This invention relates to thin-film read-only memory (ROM) devices comprising an array of open-circuit and closed-circuit cells formed on a substrate, each closed-circuit cell comprising a thin-film diode, particularly but not exclusively with one or more films of hydrogenated amorphous silicon. The invention also relates to advantageous methods for the manufacture of such thin-film ROM devices.
Published United Kingdom patent application GB-A-2 066 566 discloses a thin-film ROM device comprising an array of open-circuit and closed-circuit cells formed on a substrate, each closed-circuit cell comprising a thin-film diode; the array is formed from a stack of thin films carried on the substrate and including at least lower and upper semiconductor films of different conductivity type and/or conductivity between lower and upper electrode films.
The semiconductor films in the diodes of GB-A-2 066 566 are of hydrogenated amorphous silicon containing fluorine and have opposite conductivity type dopings so as to form PN junction diodes. An alternative ROM device is described in GB-A-2 066 566, in which (instead of a PN junction diode) all the closed-circuit cells comprise a Schottky diode formed between an upper metal-based electrode film and an underlying semiconductor film. An electrically erasible programmable read-only memory (EEPROM) is also described in GB-A-2 066 566, in which a thin film of reversible phase-change amorphous material is included between an upper electrode film and an underlying semiconductor film in all the closed-circuit cells.